Chemical reduction plating, so called electroless plating, is a process for depositing metallic coatings on solid surfaces by reducing metal ions present in an electroless plating bath with a chemical reducing agent dissolved therein. Major advantages of electroless plating is its ability to give metal coatings on surfaces of nonconductors such as ceramics, resins and the like without application of electric currents, and the ability to give uniform metallic coatings even on articles having complex geometric shapes of the surfaces to be plated. In addition, the electroless plating is superior in mass producibility to vacuum film deposition techniques such as sputtering and vacuum deposition since it is continuously operated with a cheap devices.
Electroless plating baths generally contain a metal salt, a complexing agent, a chemical reducing agent and a pH adjuster. In the electroless plating baths of the prior art, the reducing agents widely used are formaldehyde, hydrazine, hypophosphitc, hydrogenated boron compounds, etc.
Such reducing agents make it possible to deposit some metals such as Co, Ni, Cu, Pd, Ag, Pt and Au in the form of a pure metal or a metal-phosphorous or metal-boron alloy. For example, an electroless nickel plating bath containing hypophosphite as the reducing agent gives coatings of a Ni-P alloy, while an electroless plating bath containing a hydrogenated boron compound gives coatings of a Ni-B alloy.
However, the electroless plating baths of the prior art cannot be applied to deposit metal coatings on ceramics for electronic devices since the reducing agent such as formaldehyde, hydrazine, hypophosphite, or a hydrogenated boron compound produces activated hydrogen during electroless plating to reduce some metal oxides constituting the ceramics. In addition, it is required to incorporate such a reducing agent into the electroless plating bath in an amount approximately equal to the molar concentrations of the metal to be plated.
It is also impossible with the electroless baths of the prior art to deposit sole coatings of some metals of V, Mn, Fe, Zn, Mo, W, Re and Tl though such a metal can be deposited along with deposition of Co, Ni or Cu. In other words, these metals can be deposited only in the form of a eutectic composition with Co, Ni or Cu. For example, zinc can be deposited in the form of a Ni-Zn-P alloy, but cannot be deposited solely.
Further, it is impossible with any reducing agents of the prior art to deposit coatings of As, Cd, In, Sb or Pb by electroless plating because these metals serve as a catalyst poison.
The first report on electroless plating of Sn, which has been considered to be one of metals incapable of being deposited by electroless plating, was presented by K. Obata, T. Sonoda and N. Dohi in "Electroless deposition of Tin using Ti.sup.+3 as reducing agent" Metal Surface techniques, vol. 33, No. 8, P 17-21, 1982. This report teaches that electroless tin plating can be achieved by use of a bath containing TiCl.sub.3 as a reducing agent.
Up to now, however, there is no report on electroless plating of metals such as As, Cd, In, Sb and Pb, each of which serves as a catalyst poison.